1. Field of the Invention
This invention relates to an optical correlator.
2. Description of Related Art
Optical correlators are devices which automatically recognize or identify the contents of an image by combining an incoming image with a reference image, the degree of correlation after combining the images determining the intensity of an output light beam, which is detected by a CCD device for subsequent electronic processing. Such correlators currently find application in, or have been proposed for use in, such diverse fields as target recognition and tracking (including drug interdiction by aircraft type recognition), friend/foe determination, machine vision and robotics, pattern recognition (including face and fingerprint recognition), quality control and inspection, statistics collection by item recognition, inventory control, and image deblurring.
The classical Van der Lugt type of optical correlator uses a lens to perform a fourier transform on an incoming two-dimensional image, after which the light beam carrying the transform is caused to pass through an optical matched filter effectively multiplying the transform with a complex conjugate of the transform of a two-dimensional reference image. An inverse transform lens brings the resulting beam to a point or points of focus on a back focal plane of a CCD device for display or further processing, the intensity of the beam at the focal point or points indicating the degree of correlation. Correlators of this type are described in U.S. Pat. No. 3,779,492 to Grumet, and in the article entitled "Digital Analysis of the Effects of Terrain Clutter on the Performance of Matched Filters for Target Identification and Location", published in SPIE Vol. 186 and Digital Processing of Aerial Images (1979). Additional discussions concerning matched filtering and related techniques are found in U.S. Pat. No. 4,471,445 to Pernick, U.S. Pat. No. 4,490,849 to Grumet et al., and U.S. Pat. No. 4,838,644 to Ochoa et al.
The Van der Lugt type of optical correlator uses spatial light modulators to impress the incoming image on a coherent light beam for subsequent optical transformation. The electrical output of a camera is impressed upon the coherent light beam by changing the shape of a crystal through which the coherent light passes in response to the electrical output of the camera. The light beam is then transmitted through the transform and matched filter lenses. Conventional spatial light modulators are, however, in general very expensive, slow, and have insufficient image size and/or resolution.
Another category of prior optical correlator, in which the reference image is modulated by the incoming image rather than being applied to a spatial light modulator, is the acousto-optic image correlator. In this type of device, the incoming image is captured by a camera and the electrical output of the camera is used to drive an acousto-optic modulator, also known as a Bragg cell. Two-dimensional reference images in digital electronic memory are used to drive one-dimensional laser diode arrays, and the light from the laser diode arrays is imaged at the Bragg cell and then re-imaged onto a charge-coupled detector (CCD) array.
Both types of prior optical correlators use CCD arrays to capture the correlated or combined image. The CCD arrays need to have thousands of pixels. After attempted correlation, all of the pixel information must be clocked in serial fashion for subsequent electronic processing. This creates a major bottleneck and limits the number of correlations/second to a presently insufficient number, generally in the tens and in a few cases the hundreds of correlations per second. For many applications, correlation rates in the thousands per second are necessary.
In addition, both types of prior optical correlator suffer from large size and lack of mechanical stability, resulting from stringent alignment requirements, the need for focusing lenses and other light guiding elements, and the size and complexity of the respective image combining or light modulating elements used to impress information in electronic form onto a light beam.
The present invention, in contrast, does not require either the Bragg cell, laser diode array, and CCD device of the acousto-optic correlator, nor does it require the spatial light modulator, lenses, and filters of the classical Van der Lugt correlator.